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How does the Guardian counter-MANPADS system work?
I came across this picture of a FedEx McDonnell Douglas DC-10 equipped with an anti-missile defense system.

(wikimedia.org)
According to the picture description:

FedEx became the first U.S. carrier to equip its aircraft with an anti-missile defense system in 2006. The gray oval Northrop Grumman Guardian pod can be seen on the belly of this FedEx MD-10 between and just aft of the main landing gear.

How does this system work? Does this successfully work in countering all types of missiles? Can it be reused over multiple missile strike incidents? Are all commercial airplanes compatible with this system? Is it economical for a major airline to fit all of its fleet with this system?

I have worked on and designed DIRCM systems.  Most of the technical specifications on these systems is classified.  Also, the U.S. is not the only country with DIRCM systems.  In particular, Israel is equipping their airliners with the C-MUSIC system.
How does this system work?
Basically, a laser at the wavelengths of the Infrared Missile seeker is directed at the incoming missile.  It is pulsed in such a way as to cause the missile to think that the hot spot that it is tracking (generally the engines) is not in the right spot for proportional steering. Without proportional steering, the missile avionics thinks that the missile will miss the target, so it corrects for this.  As it corrects, the laser signal continues to misinform the missile as to where the target is in its reticle, eventually causing the missile to "break lock" and fly off into a random direction. Before the laser can be directed at the missile though, there are several other steps, which is why you have a complete pod.
First a missile warning system needs to pick up the fact that a missile has been launched at the aircraft (generally the job of a series of IR cameras that give a lower hemispheric view of the aircraft's environment). Then the system tells the turret to turn to a specific point in space. The turret has its own IR camera that is responsible for tracking the missile, and once it has achieved this tracking, only then does the laser start to fire.
Does this successfully work in countering all types of missiles?
This is only effective against purely infrared guided missiles (mainly MANPADs and some varieties of air to air missiles, such as the AIM-9).  If the missile has any sort of radar assist, or is radar guided, a laser will do no good.  There are some classes of laser riding missiles that could potentially be confused by these type of systems, but no anti-air types of laser guided missiles are deployed.
Many IR guided missiles do try to counter these countermeasures, so the more sophisticated DIRCM systems need to be employed. IR missiles come in many different varieties of seekers and wavelengths they use, so the DIRCM system needs to have a laser on those wavelengths.  Not only that (and this is where the secret sauce comes in) each seeker operates on different frequencies of their reticles (and reticle patterns). So to truly confuse a missile, one needs what is known as a Jam Code. These are generally classified.
Can it be reused over multiple missile strike incidents?
As long as the laser is operational, the system can be used over and over again.  Some lasers have limits to how long they can fire, and how often they can fire (the Northrop Viper laser is notoriously power limited and has a horrible MTBF).  However, as long as there is power, it doesn't run out of ammo.  This is the main advantage of DIRCM systems over using infrared countermeasures such as flares, which are limited by the number of flares one can carry.
Are all commercial airplanes compatible with this system?
The Guardian, as well as BAE's JetEye, and Elbit's MUSIC system is designed as to be integrated onto any large airframe.  Israel has already flown it on 707 and 737 airframes, and plan to integrate it to the A-320, 747, 757, 767, and even some Embraer airfrmaes.
Is it economical for a major airline to fit all of its fleet with this system?
Not really, unless one gets shot down by a MANPAD.  The DIRCM system will cost well over a million $US (probably closer to two).  And then there is the aerodynamic penalty of hauling a 350-500lbs pod on your belly.  The concept Elbit is employing is to install aircraft with a B-kit (an interface plate and the necessary wiring).  Then the airline can buy a limited number of A-kits, and install them only on the aircraft that are going to an area that may be dangerous. Notionally, this operation can be done in an hour.
As to the danger, all I can say is that over 500,000 MANPADS have been manufactured, and many thousands are in the hands of people that wouldn't mind causing a "problem" for anyone they can inconvenience. Thankfully, the training required is a bit more than firing an AK-47, so most attempts have been unsuccessful.

"Thankfully, the training required is a bit more than firing an AK-47", and so are the maintenance requirements of MANPAD systems, making them even less likely to be successful in the hands of "irregular forces" unless there's a steady stream of replacements. Which gets made even worse by the limited shelf life of the missiles. Solid rocket engines, even when stored under perfect conditions, tend to develop cracks in their engine fuel blocks, making them prone to "catastrophic failure" when fired (iow, they blow up in your face if you try to fire one that's been improperly stored for long).

<> that's why there's no risk of the hundreds of FIM-92 systems supplied to the Afghans in the 1980s from now being used against airliners, they're simply no longer serviceable.

@jwenting not to say they won't try!  The MANPADS used in the Mombasa incident against an Israeli airliner were beyond shelf life, but they launched (they just didn't hit the target)

@LarianLeQuella thanks, this is a really great explanation!  And I didn't know about that Israeli attack: http://en.wikipedia.org/wiki/2002_Mombasa_attacks

How does this system work?
Guardian is basically a Directional Infrared Counter Measures (DIRCM). By using an infrared laser (and an optical parametric oscillator to generate a second IR Band wavelength)  the Guardian jams the IR seekers of incoming Infrared missiles through their sensor apertures.
Does this successfully work in countering all types of missiles?
No, it doesn't. Against Imaging Infra-Red (IR) missiles and missiles with Additional Detector Wavelengths (e.g LWIR) DIRCM will not work.
DIRCM systems like Guardian are completely ineffective against radar-guided missiles like  9М38M1 fired by systems like BUK-M2.
Work is now underway in developing Fibre Laser DIRCMs that can counter those missiles which present day DIRCMs are unable to counter.
Are all commercial airplanes compatible with this system?
Virtually all models of Airbus and Boeing can be fitted with a DIRCM Pod like the Guardian.
Is it economical for a major airline to fit all of its fleet with this system?
No. This is an extremely expensive technology, so a majority of airlines will not be able to afford this. Remember, they have to fit this DIRCM pod in not just one or two aircraft but more than a dozen aircraft.
The most important thing to remember is that if the anti-missile defense system is overwhelmed by a salvo of missiles there is very little that it can do.

Ad "…counter those missiles which present day DIRCMS are unable to counter": It will still not be able to counter radar-guided missiles like the mentioned 9M38M1, though. That would require a separate system operating in microwave range.

@D_S I edited your post for typesetting. In English, we don't put spaces before punctuation and it looks really weird, to the point of actually being hard to read, when somebody does. Also, the plural of "aircraft" is "aircraft".

@  David Richerby - Thanks for the help . I appreciate it .

I'm curious if installing one of these changes the aircraft's 'designation' as a civilian non-combatant to something else? What I mean is that if your airliner is completely unprotected and is shot down, then your country can exclaim "what are you doing! you shot down our unarmed civilian transport, carrying tourists from A to B! (aka the recent major incident.)" However, if your airliner has protected itself using one of these, I wonder what the legal ramifications are?

Installing DIRCMs does not change the designation of aircrafts from civilian to non-civilian. Israeli civilian aircrafts have been using DIRCMs for quite sometime.Using DIRCMs like guardian does NOT guarantee 100% protection. For that matter even if civilian aircrafts carry chaffs & flares they still are not immune to Surface-to-Air Missiles . The best thing a civilian airliner can do is to stick  to commercial flight paths , avoid contested airspace and keep its civilian transponder on .

@D_S Thanks for the good answer!

@shortstheory - Cheers

and quite apart from the purchase cost of the units there's the additional maintenance cost (including crew training), and the additional fuel burn both because of the increased weight and drag of having them installed.

@jwenting with the shown system, there's not really any additional crew training. It's designed to operate with no crew input, so presumably there's a circuit breaker, an on/off switch, and an indication if it successfully shot something down, but it sounds like there's nothing they really need to train the crew to do.

@raptortech97 not talking flight crew, but maintenance crew... Someone needs to do maintenance on the things, regular checks, etc. And installing/removing as needed.

wikipedia has a brief description of the workings of it:

Guardian is designed to operate autonomously, without input from the
  flight crew. An array of sensors detects a missile approaching the
  aircraft and passes this information to an infrared tracking camera.
  The system's computer analyses the input signals, to confirm that the
  threat is real, and then directs a beam of an eye-safe infrared laser
  at the incoming object. The laser is intended to introduce a false
  target into the missile's guidance system, causing it to turn away
  from the aircraft. The detect-track-jam process lasts for two to three
  seconds. The system then automatically notifies the pilot and air
  traffic control that a threat has been jammed.

From this I can deduce that it will only work infrared targeting systems. It is fully reusable as long as the laser isn't damaged.
The unit weighs 250 kg and costs a million dollars, and the manufacturer estimates that the costs can be offset if the ticket price raises a single dollar.

Is it economical for a major airline to fit all of its fleet with this system?

That depends entirely on the airline. If an airline operates only in areas where MANPADS attacks are common, then it might make economic sense to fit countermeasures. On the other hand, if some or all of the airline's planes spend their whole lives within, say, the USA, Canada or the European Union then, at the moment, it is not economical to fit even one aircraft with an anti-MANPADS system since nobody is currently firing MANPADS at planes in those areas.
It seems that FedEx has this device on only a tiny proportion of its fleet: the Wikipedia article on Guardian mentions them installing it on nine aircraft.


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How does an alpha (AoA) vane work?
The alpha vane (also called AoA vane) is an external probe used to measure the angle of attack. I have been trying to understand how exactly it works, but I can't find any clear explanation or simulation.

Is the vane static or dynamic i.e. does it rotate along its central axis?
Given that it has a significant surface area, I think that it would either:

Rotate because of the force/drag exerted by the airflow, and give an angle of attack proportional or equal to its angle of rotation
Measure the force being exerted on it via a force sensor embedded in the surface

Is either of these correct?

In short, how does an alpha vane work?

Not exactly answering your question, but related: http://aviation.stackexchange.com/questions/2094/how-does-an-a330-detect-stall-without-airspeed

From what I have studied at my instrumentation course, it rotates and reads the angle between the current position and the reference position.
You then have several (3-4) of them around the fuselage because a roll maneuver (for example) will alter the measurements asymmetrically and in this way you can compensate for this effect.

I didn't realise there were more than two (usually) on aircraft. So, during a roll, if one of the vanes stops working correctly and provides faulty readings, that would provide an incorrect angle of attack to the FMS? (Since as you say, multiple vanes are cumulatively used to estimate the actual alpha).

@AsheeshR actually during a roll one gets a higher AoA while the other gets a lower one,

@ratchetfreak Say one freezes, and the other provides a lower reading, then the average? would be lower than the actual angle. So, now, any stall warnings based on alpha that would have been there had both been working, won't come on?

@AsheeshR It depends, you can also cross-check with the IRU data

@Federico Wikipedia tells me the ADIRU takes input from the pitot-static system to estimate alpha, which is dependent on the alpha vane.

@AsheeshR it depends on the pitot, you can also estimate alpha only with special pitots (5 holes ones instead of single holes). You asked a general question on that kind of sensor, not on how it is done on Airbus planes

@AsheeshR each sensor is checked for plausible input.  If they are deemed faulty their signals are ignored.

the ethiopean air 737max crash report reads "Crashes occurred after erroneous readings by a single angle-of-attack (AOA) sensor" ... plus Even though the jet has two AOA vanes, only one is used to trigger MCAS on a flight.

An AOA vane (like what you have shown) works by aligning itself with the local airflow, like an arrow.  The angle to some reference line (normally aircraft fuselage horizontal) is then measured with a potentiometer/RVDT/etc.  This is the local angle of attack.  Often the local angle of attack is converted to aircraft angle of attack through a calibration curve.
Most transport airplanes have 2 vanes.  If either vane shows an angle of attack that is approaching stall, the stall warning will alert the pilot(s) that stall is near.  If both AOA vanes are at the stall AOA, a stick pusher will activate to alleviate the aerodynamic stall.
The AOA values are never averaged.  Depending on the AOA computer, logic is used to determine if one of the vanes is giving bad data.

How can you measure angle of attache, which changes on the wingspan, from the body sensor?

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Why haven't quadcopters been scaled up yet?
Why are quadcopters not flown by human pilots yet?
Wouldn't they be more stable and easier to control than helicopters?

Who says they haven't? http://en.wikipedia.org/wiki/Quadcopter#Early_attempts

Good point. I'm not really asking why nobody has tried, but rather why it hasn't been more adopted yet, though.

Some serious development seems to be happening :) – http://www.gizmag.com/go/4645/picture/15365/

If the earth's atmosphere were thicker (or we had weaker gravity), then it would be more feasible -- c.f., Avatar.

Autos are difficult enough with one rotor, but the more inertia in the blades, the easier they are. With a bunch of small rotors, autos would be very difficult.

Says who?

Adding more than a single motor, mathematically increases the likelihood of failure. Failure of a single motor on the quad design would result in a crash. There is a form of computer compensation that can be employed on small scale, but it has never proved reliable at larger scales. More than a single rotor failure is unrecoverable.

Here is a human-sized quadcopter. I believe a couple engineers died during initial testing http://techcrunch.com/2016/01/06/the-ehang-184-is-a-human-sized-drone-taking-off-at-ces/

Wouldn't they be more stable and easier to control than helicopters?

No, they would not.
Quadcopters don't have any special inherent stability. When you increase power of one of the rotors to pitch, the increasing pitch will not do anything to the power difference and therefore the pitching moment.
The advantage of quadcopters is that the rotors can be fixed pitch while single (or double) rotor helicopter needs complex control mechanism. While this is huge advantage for the small scale devices where each rotor can be powered by its own simple electric motor, the complexity of either additional engines or long transmission shafts would outweigh any advantage from the simpler rotors in full-scale vehicle.
And why can't full-scale helicopters use electric motors like the small ones? The reason is that when you scale an airfoil up, the lift it produces increases with its area, which grows with the second power of size, but its weight increases with volume, which grows with the third power of size. Therefore models have much more lift for weight and can afford simple but relatively heavy batteries while full-size aircraft need propulsion systems with higher power density.
And then there is also the factor of safety. In case of power failure, helicopters can still glide to the ground and still land vertically using autorotation. But since the rotor rotating speed can't be changed without power, controlling the helicopter during such manoeuvre requires variable pitch rotor. So there goes the main advantage of quadcopters.

Actually, the area/volume argument is an argument why large copters should be quadcopters. To produce equal lift, a single rotor would need the same area, which means the rotor blades have to be twice as long as for a quadcopter, and thus 8 times the weight. And on a quadcopter the rotor tips will stay subsonic quite a bit longer as well.

Having the center of lift above the center of mass doesn't increase stability, this was the fallacy that lead to the strange design of the first Goddard rockets where the engine and nozzle was mounted above the COM on the frame.  The problem is that as the object rotates, so does the thrust vector.  The only reason the rotor has to be above the COM is if you put it any lower, it would rapidly disassemble objects/people in its immediate vicinity

@pdel: Hm, yes, you are right. I deleted the incorrect bit.

While having the center of lift above the center of mass doesn't increase stability, having the center of drag (the rotor disk) above the center of mass does increase stability.

I was always under the impression that quad copters were more stable because they could be designed with the equivalent of dihedral by angling the rotors inward.  Does that not work as well as I thought it did?

@CortAmmon -- angling the rotors inward does not give you a dihedral effect al la fixed wing aircraft -- it just decreases the rolling moments produced by the rotors a little and couples a rotor into with a horizontal force.  As evidenced by a recent crash I've had, quads are neutrally stable at best.  Any stability evidenced in flight is due to their flight control systems making up for this.

@CortAmmon, angling rotors may give you similar effect to dihedral, but keep in mind that dihedral does not give you stability in roll. It gives you stability in side-slip and yaw-roll coupling. And quadcopters don't have vertical stabilizers to give them the other half, roll-yaw coupling, to complete the stability loop for roll (second order, with dutch roll as oscillation mode).

@CortAmmon As someone who as personally built, flow, and tuned quadcopters, there is next to no inherent stability. Pretty much all of a quadcopters stability comes from a flight controller (ie computer + gyro combination) continuously adjusting the individual motor speeds.

@MSalters There is more to it than just total area. A single large rotor is still more efficient than four smaller rotors with the same total area. If tip speed is too high to increase rotor diameter then wou would be better off adding more blades to the larger rotor than splitting it up into multiple smaller rotors.

It has (sort of) been done:

The company that did this is working on a more useful version.
It's important to note that scaling up is extremely difficult in aviation.  Model airplanes have performance numbers the full scale folks can only dream about.

I'm hoping an aeronautical engineer can chime in on the challenges of scaling up a model.  I think Reynold's Number is involved, but I don't know enough.

It's easier than that. Lift is proportional to area, mass is proportional to volume. When scaling up, the latter grows faster.

Video of the Volocopter https://www.youtube.com/watch?v=RUGfXD0SW5Q

@BrianWheeler: Unfortunately comments are not editable (are editable for 5 minutes only).

@JanHudec that is unfortunate. I feel like that was a recent addition to the SE network... thanks for trying!

See Flight of the Phoenix.

Is.. that an.. exercise ball?

Doesn'tlook to safe, ex

@easymoden00b Looks so. Seems like a good "landing gear" :)

Ah, the elegant, simple, beauty of flight.

Quadcopters are not an efficient design -- one large rotor is far more efficient than four smaller rotors.  The reason quadcopters became popular is that they are mechanically simpler, safer (due to the smaller rotors) and far easier to control by software.
That said, recent advances in machine learning have made so that helicopters can be controlled by software.  I'd expect to see a resurgence in helicopter-style drones because of this.
Finally, I'm not an aeronautics engineer but I suspect there's considerable merit in combining attributes from a quadcopter and a helicopter.  Imagine one large fixed-pitch rotor driven by an electric motor.   A few small additional electric-motor driven rotors could provide the needed pitch, yaw, and roll control.  Electricity would be generated by an efficient gas engine spinning at near constant speed.  Enough backup power to land could be provided by small batteries.  This design would be extremely reliable due to the greater reliability and simplicity of motors and fixed pitch rotors.
Advancements will come, but slowly since the costs and dangers of human-carrying vehicles are so high.

This needs to be emphasized. Four smaller rotors totaling the same area are LESS efficient than larger single rotor of the same total area.

Advanced Tactics has also somewhat did it!

A video of the first flight of the Black Knight Transformer has been released on Youtube.

sheeeshh that thing's ugly

This needs an entry here https://en.wikipedia.org/wiki/Category:Eight-engined_aircraft

Ah, the elegant, simple, beauty of flight.

Four rotor copters were actually the first copters...
Raúl Pateras Pescara, Buenos Aires, Argentina, 1916

Etienne Oehmichen, Paris, France, 1921

This doesn't answer the question.

Well, actually, it does. Not the intent, I suppose.

There are three challenges for making a market-competitive full scale quadrocopter.
1) Energy efficiency
The quadrocopter is not an energy-efficient design. Helicopters increase their range by using relatively under-powered engines that take a long time to spool up throttle and try to keep it at relatively constant RPM while changing the pitch of the blades to control the helicopter. While this can be used to control pitch and bank, the quadrocopter controls yaw by changing the RPMs of the motors, which I would think would be difficult, and inefficient to do on a full scale helicopter without some sort of CVT to adjust the RPM of the blades without changing the RPM of the engine.
2) Structural integrity and weight
Tail rotor helicopters essentially have a fuselage and a tail boom. The tail boom doesn't even need to be that beefy, because the load on the boom is rather small compared to the load of the main rotor pulling up on the frame of the fuselage. Coaxial helicopters can probably get away with just a fuselage, and no tail boom, and probably have less forces acting on the entire structure in normal operating conditions than conventional helicopters. Quadrocopters have four main rotors. Granted, each of the rotors is approximately 1/4 of the power of the main rotor of a conventional helicopter, but the booms would still need to be able to withstand more than 1/2 of the power of a conventional main rotor - think of the extreme case: you're full yaw - that means two rotors diagonally across from one another are either off, or spinning in the opposite direction, creating either loss of lift, or even down force that the other two rotors have to compensate for by producing two times the thrust/lift. It's a rather common phenomenon among miniature R/C planes and helicopters to be completely overpowered, and much beefier than their full-sized counterparts.
3) Cost of manufacturing
It's worth mentioning that a larger rotor is easier, and cheaper to manufacture than three additional engines/motors. I think (I could be wrong) the only reason military helicopters have two engines instead of one big one, is for redundancy. If one gets shot, you still have the second one that you can use to limp back to base, or a friendly location. So when you're looking at a whopping four overpowered engines (see point #1) compared to 1-2 underpowered engines, I would imagine you would be looking at something that costs more.
Those are the reasons why a quadrocopter has not been scaled up, and become commercially available by a large manufacturer. I'm sure each manufacturer is trying to be the first in that market, but I don't think QUADROcopters will ever become human-piloted. If you were talking about making a MULTIcopter (more than 4 rotors) however, there are a few DIY prototypes out there, and I think they have potential.
4) Addressing the proposals for hybrid systems:
As things stand now, conventional helicopters run 1-2 turbines for power. They spin in their optimal RPM ranges, and the pilot uses the collective to change the pitch of the rotor blades to control flight. A hybrid system is appealing for cars because it allows the ICE to run in its optimal RPM range rather than constantly changing RPMs. In rotorcraft this isn't an issue - since they already spin at optimal RPM ranges, a hybrid system would introduce a generator, and batteries to generate and store power for the electric motors to consume. This would add weight, without any particular added benefits.
The big question is - why bother? What makes quadrocopters better than conventional helicopters? For drones the biggest benefit of quadrocopters is their ease and cost of production. It's cheaper to produce four small electric motors than all the mechanisms for a collective. It's also more robust and easier to service. However, we're talking about different scales and missions. Rotorcraft are already inherently inefficient. Their niche - close air support, and transport to locations where it's difficult to land an airplane. The tendency is to create a hybrid, or make a plane with VTOL capabilities. (Osprey, Yak-141, Harrier, F-35 aka US Yak-141). Quadrocopters may find their niche as cheap utility drones, or surveillance drones, but I think a much more likely design, is the likes of the one shown in Avatar - two counter-rotating shrouded rotors on either side of the fuselage, which can be vectored to control flight.
Technology is ever-changing. Scientists in Russia have miniaturized nuclear reactors and fit them inside missiles. This negates the need for batteries and hybrid systems. If anything, this is the powerplant of choice for large quadrocopter systems, but only time can tell how technology will evolve. Right now, I don't see the benefit of quadrocopter configurations for large rotorcraft.
Amendment, based on new tech in the market (23.03.2018):
A Chinese company is actively marketing their EHang 184 autopilot drone air taxi, that uses the quadrocopter design.
Specs:

100kg payload
25min cruising time
1hr charging time
500km/h cruising speed (very curious to see this)
500m cruising altitude

Basically, one can compare it to conventionally designed Mosquito helicopters. They fly slower, but duration of flight,range, and useful load are significantly higher. Also note that while fueling a helicopter these sizes takes five minutes, the air taxi will spend 2 times longer charging, than it will flying. But this is more of a EM vs IC type issue. Time will tell if quadrocopters can prove they're worth up-scaling.
Adding from recent professional experience (09/19/2018)
I was recently asked to develop a quadrocopter concept. While attempting to use electric power, I found that payload weight, and mission flight time drastically increased the weight of the batteries required to fulfill mission parameters, and you ended up with a 3-4 ton four-seat quadrocopter (yikes!). Converting to turboprop solved the weight issue, but dramatically increased cost and complexity. As a result, the decision was made to drastically reduce mission parameters, making for a very short range vehicle, which under normal circumstances would be quite useless. You would need to charge the batteries for several hours to get 15-30min flight time. Either your quadrocopter would be sitting on the tarmac charging, or you would need to swap batteries after every hop, requiring expensive infrastructure at every landing pad. Ultimately the idea was proposed with infrastructure and battery swap, and is awaiting its fate at the hands of upper management, but everyone on our team has already scrapped the project as unfeasible.
There ARE a number of upcoming battery technologies that would drastically change the potential of such products, but they're not market-ready yet, mostly in theoretical, conceptual, and testing phases at the moment. Can't say much more on the subject without an NDA, sorry.

Cost is what I'm thinking.  Certified piston helicopter engines can cost $50,000. Even the smallest ones are in the $25,000 range. Now multiply that by four:  You've quadrupled the cost of one of the most expensive components for no gain.

Electric engines are amongst the most efficient in changing energy into mechanical power, it are also lightweight; of course, you can have an ICE, e.g, a Wankel RCE, driving a generator to a small, emergency and reserve battery, this combustion engine can work always at its best SFC rpm range. There are many simple ways to control rpm of an electric engine. That the concept proved practical has the backing evidence of the lots of man carrying size quadcopter prototypes existing today, just look on YouTube.

@Urquiola The #1 enemy of all aircraft is weight. You say Wankel - I say turbines have better power-to-weight ratios. You say hybrid system? ut That means you have to have an 1) ICE that consumes 2) fuel, to feed a 3) generator, that stores energy in 4) batteries, so that four electric motors can turn rotors. Why all the extra parts and components?
Technology changes, maybe one day we'll get to the point where technology will make quadrocopters reasonable for production, ut we're not there yet.

I was wondering about your statement that it is necessary to change the rpm of the blades in order to pitch, yaw, roll etc but similarly to a plane wouldn't changing the blade's pitch as done in a fixed wing prop provide a more efficient and simple fix?

Not for yaw, for that you would need cyclic. While variable pitch props would solve pitch, roll and lift, it also adds complexity to all four propellers, and variable pitch propellers are more expensive. This all comes down to what is cheaper.

Scaling up the quadcopter design would make them very large. Only having one (or even two) rotor allows the helicopter to be smaller and even fold up the rotors and remain fairly compact. Since by definition helicopters are supposed to get into more difficult landing areas, increasing the footprint is generally undesirable.
Also, the power system would be complicated. RC quadcopters use an electric motor on each rotor for independent control. A larger version would probably use turbines like most helicopters, and they would either need 4 turbines or a way to gear fewer turbines to 4 rotors. This just adds complexity to the system.
The Chinook is an example of a helicopter that uses two rotors, but the added complexity makes it undesirable unless the added lifting power is needed.

and anything large enough that needs to be lifted by 4 rotors would also be large enough to intercept enough downdraft to make any helicopter lift difficult

also the increased speed capability of the chinook

Below is the info about one of the early prototypes by Paul Moller, author of the 'Volantor', too, now the head of Freedom-Motors, a company focused on Wankel Rotary Combustion Engines. Moller built his flying machines with fans driven by Wankel rotary engines, however, it sounds better having electrical engines for lifting and  propulsion fans, and some kind of hybrid system for generating power, Wankels are very good in weight to power ratio and safety. I'd say the Moller and Helikar are exactly a Quadcopter, and as this machine, and the Utility model/patent backing it are from earlier times as the toys we all know, probably it was the chicken of contemporary toy Quadcopters, the today toys being the eggs.
The rope and the crane behind the Moller: 'flying saucer', Avro Canada failed in having one airborne, are not for keeping the machine in the air, but a safety measure, if you look, you'll notice that the rope is not under tension while the machine on the air, meaning that the machine flies by itself, and is not hung on the crane.
The prototypes Bell Textron/X-22 and Curtiss-Wright X-19 can be considered also very close to a quadcopter.
A Spanish team: FuVex, designed a prototype named: 'HeliKar', also close to a big size Quadcopter. YouTube includes a video about a German invention named: 'Volocopter VC200', Terrafugia proposed another 'Flying Car', as well as Zee. Aero did, they draw a concept VTOL flying car that could be parked between two ordinary cars in a mart parking lot (see: 'SlashGear'). Aero-X by Aerofex is another one. Fipsi is involved in developing a four-fan flying car, as cited in 'Advanced System Engineering', SUSB Expo 2014.
'Future transportation' cites what they consider: 'The top 10 flying cars'. Krossblade proposed the SkyProwler and the SkyCruiser, the big problem in a flying car seems fulfilling in the same design the weight and safety requirements for both a car and an airplane. Ford released drawings for an: 'Aero-car'. CNN reports about Fly Citycopter, by E Galvani.
Mechanix Illustrated, March 1957 includes a: 'Flying saucer', design by Peter Nofi, power comes from a flat-six reciprocating engine, and Popular Mechanix, French editon, Sept 1961, a similar design: 'Rotavion', by Ben Kaufman.
Mechanix Illustrated, Jan 1962, pp 70-73, is about a VTOL device having 3 shrouded fans moved by a turbine.
Regarding historical precursors, the machine in the 1961 movie by William Witney: 'Master of the World', based on two novels by Jules Verne: 'Master of the World', and: 'Robur the Conqueror', could be easily considered as a Multi-propeller: 'Quadcopter'. If: 'Fly by wire', succeeded in keeping airborne intrinsically unstable and unsteerable designs, there can be no limits in what could be accomplished with a digital control technology that currently is sold in toys starting at around $20.
A Hungary based company: 'Flike', and Hungary's name doesn't come from 'Hunger', but from: 'Huns', just presented a 'piloted' three propeller copter, video is signed by Bay Zoltan.
Do the flying platforms, single person devices, ducted fan below, tested by the US Army, belong to the: 'Quadcopter' class?. Also, several 'tilting wings' and annular wing flying machine arrangements were tested, the list of 'Concept' and 'Unconventional' aircraft paves the way for many books, but the web is killing this type of paper publications.
'SkyProwler', is another approach, a mixed Quadcopter and fixed wing design.
Bensen B-12 (see 'Aerofiles') is a 1961 example of Multi-Rotor (rather multi-propeller) Quadcopter style machine, patent US49820151 is about the multi fan P. Moller's design, and patent CA1264714 is about a: 'Robotic or remotely controlled flying platform', also by P. Moller. All patents are of open and free access and download in: 'Espacenet'
Is there a reason why Quadcopter toys couldn't be increased in size, even using the same software for controlling engines and stability?
Thanks.  + salut
Moller M200 test flight
Helikar
P2 Hoverbike from Malloy Aeronautics

what's the usefulness/meaning of the pictures you just added?

CGC Campbell stated below that enclosed fans would loss function. This is to point his assertion is not fully accurate. The Wing buried Jet Engines in Northrop Flying Wing, also in De Havilland Comet, fulfilled its task too. Federyco: ¿Fondo Europeo de Desarrollo Regional y Comunitario? Gesund +

If those should be an answer to a comment, they should go in the comments. If those are part of the answer, please add an explanation to the answer around the pictures. Right now they are just noise.

They have been scaled up - perhaps not in rotor size but in number of rotors. If this can be done, I can see no practical limit to producing a manned platform with four rotors.
Controllability seems to be a bit of an issue, but maybe that is because the pilot has mounted a milky glass dome around his head.

Ah, the elegant, simple, beauty of flight.

"but maybe that is because the pilot has mounted a milky glass dome around his head."--  that is a hood for blind flying practice

The reason quadcopters are chosen as platforms for small, computer controlled flying machines is because they are by design more agile and simpler to move in and around all three axes. This is because they are by design very very unstable.
It is in fact impossible for a human to control a quadcopter (that can move in and around all three axes) without the aid of a computer or some other kind of artificial stabilizer. The reason computers can control quadcopters is because they are fast enough to produce control input that counters any small destabilizing forces that act on the frame.
One way to imagine the inherent stability of a flying platform is to consider what would happen if you would let go of the controls. Normal airplanes and helicopters will tend to just keep flying to the same direction. If you let go of the controls of a quadcopter (and have no stabilizing mechanism installed) the quadcopter will very quickly just chaotically tumble towards the ground. This means that in a helicopter or an airplane the design "helps" you and forces the frame to fly in a stable manner (forwards.) In a quadcopter there is no such help but there are also no forces that hinder you from moving towards any direction you want.
This principle is also deliberately on frames such as the F-117. The F-117 is impossible to fly without the help of computers (it was designed to be unstable), but this has made it a much more agile airplane than its frame would normally have allowed.
The other (bigger) reason quadcopters have not been scaled up is because they consume (a lot) more fuel than other types of aircraft. Why would anyone build a quadcopter when a helicopter or an airplane does the job using less fuel. They are also slow and noisy.
Keep in mind that the "job" for scaled up airframes is usually to move stuff from point A to point B while the "job" for small, computer controlled quadcopters is to be agile.

Quadcopters can be stable, neutrally stable or unstable. It depends on whether they have centre of gravity below, coincident or above of centre of lift. Most are built with neutral stability.

I wouldn't be so optimistic on the natural stability of helicopters. Unless it is a Kamov with two coaxial rotors, helicopters are unstable. However, it is easier to control two rotors than four.

There's a number of comments above about the various reasons that quadcopters don't exist in full size, but there's one I don't see mentioned. The quadcopters themselves use 4 rotors that have a constant pitch (the angle of the rotor blades is constant, and doesn't change). It then varies the speed of the various motors to control the lift provided by each rotor, so that it can coordinate flight. At a small scale, it is possible to use both an electric motor, and additionally, to adjust the speed of the rotors very quickly. On a normal, human sized helicopter, the rotors would need to be significantly larger. Once the rotors get scaled up, they also weigh more, which gives them more inertia than a smaller motor. For example, in a Robinson R-22, which has relatively lightweight rotors and can only carry 2 people, it takes me something on the order of 3-4 minutes for the rotors to stop spinning once I remove power (on the order of 30 seconds to a minute if I'm using the rotor brake). As the size and weight of the helicopter scales up, this problem gets more significant. You're unable to quickly adjust the speed of the spinning rotor because of the inertia it carries, so you'd end up having to add something in addition that lets you control the pitch of the blades. Once you've added the mechanical complexity of being able to control the pitch of the blades, most of the advantages of a quad copter setup go away, and you have significant disadvantages (like having to deal 4 rotors instead of a single one, without the advantages of mechanical simplicity).

There is a lot of aerodynamic resistance at high rotor RPM that does a good job of slowing it down.

As I mentioned in my comment above, it's not enough to overcome the additional inertia from the larger blade. A lightweight helicopter rotor that is on an R-22 (a bit more than 25 feet in diameter), at something close to sea level (about 700 feet, if you're flying near Chicago), spins at about 600RPM normally. That rotor, without a rotor brake, still takes 3-4 minutes to slow down. That is not nearly enough drag to rapidly change the rotor speed. You're additionally limited in the range you can spin the blade (typically 80%-106% of 599 RPM), and it's something that doesn't scale up well.

Drag scales up very well, and is the easiest force to add. Both small and large rotors work at their best with a tip speed of around O.8 Mach, which creates serious slow-down drag for any size blade. Drag decreases with the square of speed, and at low tip speed you have to wait forever for the large blade to come to a standstill. Like easing pressure off of the car brake as you slow down. But if drag is too low, it's simple to increase it by sticking a spoiler out.

Drag may scale up, but a rotor's inertia scales up much faster than its front facing surface area, so nonetheless, a large rotor will take longer to slow down. You're going to have a lot of difficulty speeding up and slowing down a large rotor fast enough to use to maintain stability, which gets you back to having the mechanics of controlling your blade pitch instead of velocity. My point was not that its entirely impossible, only that the quad copter setup with fixed pitch blades having varying blade speed, and mirroring that functionality on a full size blade is difficult.

e-Volo claims it will use range extender in VC200: http://www.e-volo.com/information/how-long-can-you-fly
I was really curious about that technology, and did small research. My conclusion is, if they consider using Bladon Jets micro turbine to generate electricity it could be a total win!
So, it seems, the most advanced turbine they have now is 50 kW, 40-kg jet turbine for concept car: http://www.bladonjets.com/news/bladon-jets-at-the-geneva-motor-show/
According to wikipedia, Volocopter motros consume ~36 kW (18x2 Kw, see E-volo_VC2)
which means, they can be powered directly by electricity produced by turbine without a need for batteries! Of course there must be small battery pack which provides energy buffer for safe operation.
There is also a datasheet on bladonjets website about the fuel consumption of stationary 12 kW turbine, it's 5 liters/hour. Let's say, their 50 kW turbine will consume 5 times more. Which means, 50 kg of fuel will be enough to fly vc200 for at least 2 hours (not 20 minutes, as they say for pure electric e-Volo).
Also, total weight of system is not that much exceeded by turbine: 40kg turbine + 50 kg fuel + 50 kg chassy. Well, e-volo can alredy carry 2 people=150 kg, right? So existing prototype can lift the generator as is. 50% stronger motors (18*3 kW=54 kW) is not a problem to lift another 150 kg. Although total weight might exceed 450kg - category of ultralight rotor aircraft, but who says that specially designed turbine can't be integrated in e-volo structure?
My point is, the technology is there. Just apply it alltogether, and it will make a revolution.

For clarification on some of the answers, technology in the area of distributed propulsion is moving towards systems that would allow hybrid quadcopters to be built that eliminate the problems of a gas quadcopter and remove the energy density limitations of electric systems. I suspect we will see experimental human scale quadcopters in the next 20-30 years.

The only answer that matters is safety, because it makes all the others unimportant. Assuming fixed pitch rotors, if you Lose power to a rotor, you're going to crash, and you will crash in an uncontrolled way.
You could gain more control by adding more rotors, but the more rotors you add, the higher the likelihood of a failure of one rotor. Also, for each rotor you add, you'll lose rotational inertia in each rotor, the result being that in a loss of power event, even collective pitch rotors will be unable to provide sufficient lift to keep you from dying of rapid deceleration sickness.

Quadcopters are the kind of flying machine that needs having one, or two emergency parachutes to be flown in confidence. Do it? I won't seat in an human-size Quadcopter with unprotected propellers at the level of my legs, belly, chest, or head, in the last days of Radial engine airplanes, having a propeller going on its own, detaching from propeller's hub, and cutting the cabin, was a repeated event

In my mind there is only one thing standing in the way of this form of transportation.
Power density

There are heavier-than-air firefighting (mine service, etc.) lift frames with four lift props. Ones without whose adaptations and use cases one would find rides:
Pitchy; trim requires pitching or rolling the crew, or at least jinking the evacuee 'hard mount.'
Blendy; Four sources of chop, no particular service cowling (emergency fixup, water landing kit, etc.) in air or not, or guarantee of no inflow at ground.
Noisy; reaction lift is from outer extent of blades; perhaps orbit of blades can be altered to optimize pattern?
Harder to service-harden by 3x or more (for commercial suitability, nevermind commercial war duty. Dual-engine certified? Sorry.)
The e-volo multirotor movie has a nice white seemingly unloaded 2-fore heli frame with a DC patch panel, no rotor or tail, and instead a rack of smaller props. It goes up in a test hangar/messe and doesn't self-invert or evert...at some point reminds of the incidental score from that drone repairman flick where Jony Ive designed the humankind-wrecking drones' looks, and sounds like a lot of fans; performance is not up front in this production (though batteries were surely lofted.) They should at least cut the reel to appear to have a white cat onboard?

Hello Mahembo, welcome to aviation.se. Can you edit your answer to make it more readable, it is a bit difficult to get through at them moment and doesn't really make sense this way.

A hobbyist as me may easily come to the conclusion that systems that control a toy size quadcopter: gyroscopes, hardware/software that regulates electric engines' speed, would be the same for a toy and for a person-carrying flying machine, and considering the extemely low cost of the electronics in quadcopter toys, I wonder why no news exist about somebody having tested yet the toy hardware in full size machines. Comments?

Noise and Safety
Noise - If you live in a big city, take a look out nearest window at the traffic snarl below. Now imagine all those cars (or at least half) replaced by flying human-sized quad-copters. To scale up, those quads will need piston engines (or expensive turbines) and large rotors, and if successful there will be literally millions of them in the sky above any medium to large city. It would be a constant, deafening drone which would significantly reduce the livability of the city.
The Black Knight Transformer mentioned by @menjaraz is very, very noisy, as are conventional helicopters.
Safety - Unless you're a qualified pilot, you can't manually operate an aircraft. This means they'll need to be able to take off, fly and land fully automated to their destination, navigate along the way, communicate with and avoid other traffic and have some kind of interaction with ATC, all done for you by computer(s).
Both these problems are surmountable, but will require significant R&D, which means investing big dollars in a shakey economy. And who's going to buy all these multi-million dollar machines?
TL;DR: The limitations on human scale quad copters are primarily economic, regulatory and environmental, but those are significant limitations.

The question asks why there aren't person-sized quadcopters, not why person-sized quadcopters haven't replaced cars.

Some knows about the providers of electric engines for all these prototypes? I have some ideas, too

@Dave There are person-sized quadcopters, they're just not practical (yet).

@Urquiola The electric motors are not the engineering problem to be solved, it's the power source. Batteries have a low energy density to weight ratio, and the energy required to lift you off the ground increases by the square of the gross weight of the vehicle. A battery powered human sized quadcopter using today's technology would have 10 or 20 minutes of airtime on a full charge - not practical!

PS. Dubai Police are testing a "quadcopter motorcycle" for rapid response. Take a look: https://hackaday.com/2017/10/16/dubai-police-test-quadcopter-motorcycle/

I'd say because Flying machine engines work at full power just in special parts of mission envelope, such as take-off and climbing; the required power for level cruise flight may be in the range of half the top nominal power (see: 'Power struggle, why car engines won't fly', Smithsonian institution), a Turbine will be too thirsty under such conditions. Electric motors are safer, lighter, allow more placement choices. You don't need but a small battery to buffer power, and act as reserve, not as big as in a pure electric car. This concept may be good for 'Tilting Rotor' machines. Is it?

The poor weight to power ratio of batteries is one of the reasons why I propose a Wankel RCE as Electric Power supplier, added to an small battery. Rotaries are light, safe, economical at constant rpm; its use in an escalated Quadcopter would be same as Range Extenders for cars. Please, do not answer to things I didn't say. OK? Salut +.

I don't want an electric engine for a Quadcopter project, but for an updated version of Custer Channel Wing, with propeller disc over thickest airfoil part. Retail prices for electric engines suited for Rogallo Wing type of machines are expensive. Thanks, Salut +

That barrel wing is an interesting concept, buy it's a STOL aircraft, when we're really wanting a VTOL solution.

I think that both single rotor than quadcopters will cohesist, at least for some time. The reason for creating manned quadcopters are countless. A quadcopter could be driven like a car, while real helicopters need hundreds of flight hours for a pilot to master. this will lead to mainstream production and lower manufacturing prices to make them available to the mass.
about usefullness: a quad copter can have the blades ENCLOSED inside protecting structures, making their flight viable for cramped areas, where trees, buildings and poles would otherwise destroy normal helicopters. this makes a quadcopter much a better choice for rescue operations.
probably normal helicopters will stay a better choice for long range operations and speed.

You cannot completely enclose the blades, otherwise where does the lift come from. If what you mean is a shroud around the circumference of the blade sets, that will not protect the blades from tree branches or poles. It might protect against low speed sideways bumping, but anything faster than a very slow crawl would tend to bend/warp the shroud causing just as much damage as a normal blade strike.

Good luck trying to certify such a contraption. You would need a triple-redundant FCS just to stabilize that thing, and if you have four engines plus rotors, you will get four times as many failures and four times as much maintenance throughout the life of your quadcopter. Put your average car driver at the wheel and then prove that the accident rate is comparable to that of conventional helicopters. This will be very expensive, if you succeed at all.

The issue favoring 'full size' quadcopters may be that the controlling mechanisms: gyroscopes and the connected hardware and software may be not too complex nor too expensive, you find toy quadcopters gyroscope stabilized, 'in 6 axis' the sellers say, for less than $50, no reason why the same electronics can't drive a larger flying machine, it's just a matter of end output servos, and related parts, but the core doesn't change from a toy to a full size device. Or does it?

One thing that does change is the requirement for reliability. If a small toy crashes noone really cares much. If a craft big enough to carry a person crashes it's a much bigger deal. If there is actually a person on board then it's an even bigger deal.

Did you hear about: 'Ducted Fans'? there are many publications about this, in the range of airspeeds below 100 mph, it are better than ordinary propellers, the good range for Turboprops and Turbojets is much faster, Ramjets are suitable only above Mach 1 or higher, or in the Hyperspeed range

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why do fighter jets still have guns/cannons?
I noticed that the "new" F-35 still has a GAU-22/A cannon installed in it -- albeit with only  182 rounds (more with ammo pods).
Why do fighters still have cannons? The day of dogfighting is long over, now it's just fire some missiles and forget.  Even if they expended all their missiles, 182 cannon rounds is only 6 seconds of firing  (I think the GAU is rated to 1800 rpm).
The only reason (and I'm not sure if I should answer my own question) is that they need something to shoot warning shots over the bow of another airplane. But that seems an awful amount of weight for this rare occasion. And it'd mean loading tracer rounds.
Or is just a vestigial organ, like the sabres on the side of full dress military uniforms?

For the same reason Marines still get Bayonets to attach to their M16: Just in case you need to get up close and personal.

Up close and personal might be the norm in peace time.

And from what I remember, 6 seconds is similar to the amount of time you could spend firing the guns on a Spitfire in WW2; that's quite a lot of time if you're not indulging in spray and pray.

He's too close for missiles, Goose, I'm switching to guns!

If there was not a gun, how many more missiles could there be?   If less then 1, then the gun seems like a good ideal, if only for taking shots at the ground.

Why is the day of dogfighting over? I don't follow military aviation too closely, but even so, this does seem like a bold statement to me.

@shortstheory I'll venture the guess that WWI-style really-close-quarters dogfights aren't happening anymore because modern fighters have very very good long- and medium range weapons with a very high probability of success. It's absoultely unlikely that two fighters would come that close to each other. A fighter vs. something else though (bomber, tanker, hijacked civilian,...) is another matter but again the ranged weapons would be the better choice.

It's for the fighter pilots that really know what they're doing. This whole missile business is over-rated.

"WHAT? A hot air balloon full of Dynamite heading for Gotham? Scramble the fighter jets."
-"Errm, sir. The balloon is mostly cloth and basket reeds, our missiles can't get a lock on that"
"Arse! I knew we should have installed guns, well tell the pilots to fly directly through the balloon, lets see what happens"

As Maverick said: "Too close for missles, I'm switching to guns." http://www.imdb.com/title/tt0092099/quotes?item=qt0447372

A few questions. 1. Are gunsights dependent on radar to get an accurate distance? IIRC in the 50s radar enabled gunsights were a big improvement. 2. How much gunnery training do fighter pilots do? 3. Have guns been shown to have any effect on plane availability?

So they can blow away Cessnas that get too close to the White House.

@AndyBlankertz that should be a separate, dedicated question

Those aren't ammo pods. They're actually gun pods, containing both ammo (200+ rounds) and a gun

There is a point that is not the exact answer for the current situation, but might be part of the answer in a while. Some drones produced in the developing countries are getting good enough to cause some headache and cheaper than the missiles that could be fired at them.

The idea that missiles will be all a fighter aircraft needs was prevalent in the late Fifties. The McDonnell-Douglas F-4 Phantom II carried initially only missiles, but at the start of the Vietnam war this turned out to be inadequate. The long-range missiles back then were rather unreliable, and in a real conflict things turn out always different than anticipated. As von Clausewitz said, the plan is the first casualty of war.
F-4s frequently found themselves in close-combat situations for which they were inadequately prepared. Even today, a gun gives the pilot a lot more options, and if the situation is unclear, close-up visual inspection before shooting is still vital to avoid politically embarrassing situations.
From the F-4C on, F-4s were equipped with a gun. First in external pods, from the F-4E as an internal gun. What also helped to make the F-4 more effective was better training for pilots and ground crew, so the missiles were in better shape and better applied.

That may have been true in the 1970s, is it still true now? Missiles are much more accurate and reliable. The only target I could see a fighter shooting bullets at is an airliner, which chills the mind to imagine.

Probably a tradition by now, though. In the Gulf war, the only air-to-air kill made with a gun was an A-10 gunning down a helicopter, and the A-10 isn't a fighter jet. In fact, during the Gulf War fighters scored more air-to-air kills using bombs than bullets (!!)

Besides missile unreliability, another factor handicapped missile-only F-4s in Vietnam. The rules of engagement required that enemy planes be identified visually before being shot at, and this was impossible at missile ranges. F-4 pilots either had to violate their ROE, or make a close pass for identification, defenseless, then try and open the range back up to engage with missiles. This is still potentially an issue in modern combat environments.

@RussellBorogove: That's why we now have AWACS. BVR kills have become normal.

AWACS can't positively tell you if a non-radiating target is friend or foe, and you can't guarantee AWACS will be available when you need it.

and in a heavy jamming environment, that missile is going to lose a lot of its reliability. Bullets aren't fooled by chaff and flares, or ECM gear.

@RussellBorogove: Modern radars can count compressor blades. True, you might be in situations where both sides fly Mig-29s, but that's the exception. Also, having taken off from a hostile airport is usually a good clue.

@MSalters : because in the Gulf war there was no big aerial battle between similar forces. It mostly was one force bombing the (much worse-equipped and smaller) other force before they even could take off. Had they faced a similarly-equipped large air-force in the air, there would have probably been at least a few dogfights.

@MSalters: Counting compressor blades only works for a narrow range of aspect angles. Visual identification still has merits in many cases.

To be fair, Iran Air 655 was shot down by a boat, not a fighter, so guns weren't really an option there. Korean Air 007 is probably a better example, having actually been shot down by a fighter.

Don't forget another important aspect - missiles are much more expensive.

@reirab: That was a scenario where a fire decision was based on electronic evidence (in this case grossly misinterpreted), and exactly the same would happen in BVR combat. The platform is secondary.

@PeterKämpf Yeah, but BVR was the only option there, as a boat can't go visually inspect an oncoming aircraft. The platform makes a rather substantial difference in this case...

@Luaan I doubt that really factors much into the decision. If anything, the expense of designing the plane with the gun in the first place would most likely offset any munitions cost difference. Furthermore, they're not going to risk a many-million-dollar fighter jet (and a pilot) just to save a few thousand by firing bullets instead of missiles.

@reirab Hah, I actually did originally factor that in my comment, but decided against including it. It isn't an important factor in fighter-fighter combat, of course, but it might be a factor in different scenarios, like hijacked civilian planes or attacking other "defenseless" targets. And it's not exactly "a few thousands" - AIM-120 AMRAAM D costs almost two million dolars a piece (including development costs I assume). So the cost of the missile versus the cost of the jet is around 1:100 for a F-35 - still rather huge, but...

@Luaan Yes, but that still doesn't account for the R&D and production costs of putting the gun on there in the first place. Also, I was thinking more of a Sparrow at $125 k or a Sidewinder at $665 k than an AMRAAM. An AMRAAM would be kind of overkill for the defenseless target scenario you mention. If the pilot already has a gun and the target is defenseless, then, yeah, the cost consideration might make a difference, but they wouldn't add a gun to a plane for cost reasons.

@reirab Yeah, no argument there. It might be a bit different if the governments were a bit more trigger happy about shooting cilians down, but it isn't as important a cost reduction as that quite yet :D I'd assume the R&D and production costs might be offset by the fact that these guns do make a lot of sense on CAS craft and helicopters, but it's hard to judge that. It might even be that the plane wouldn't really benefit from not having the gun.

@Luaan Saving several hundred pounds and the design complexity needed to absorb the shock from firing the guns without damaging the airframe is a pretty significant cost saver, both in R&D and operational costs (and it's not an insignificant performance cost, either.)

@reirab Well, that's not something I could judge realistically. I'd assume that that would already be covered by the different but similar craft, like the Warthog with its "My guns produce more thrust than my engines". But I don't have any hard data either, so really, I'm not going to contest that.

Early F-4's didn't have guns. T'was a big mistake. Missiles are not the perfect air-to-air or air-to-ground weaponry. My understanding is that in the cockpit, in a combat situation, the fact that a cannon shell costs 100 bucks and the missile costs $100,000 or $1,000,000 or $10,000,000 doesn't usually come up. Pilot: "They're 13 miles out - take 'em with missiles!" NFO: "Whoa, there, hoss - hadn't we better run a cost-benefit analysis to determine the most fiscally efficient way to down that bad boy?" Pilot: "SHOOT THE #&@$# MISSILE, RIGHT #&@$# NOW!!!!"  :-)

Love your work, but I have to address this myth. Guns didn't fix the F-4 because... missiles weren't the problem---training was. When Vietnam started, F-4s achieved an abysmal 2:1 K/L ratio. So the USAF added guns. The USN didn't: they trained the maintenance crews how to maintain and handle missiles as a weapons system and implemented a new training program for pilots. USAF K/L ratio actually decreased slightly. USN K/L ratio rocketed to 12:1. That training program? It became Top Gun.

@RoboKaren - Answer1: the F35 carries a gun largely for air to ground. Answer2: it doesn't. Only the A model carries an internal gun. The B and C models carry podded guns (220 rounds) only when needed. Answer3: WVR combat has steadily lost relevance for decades and continues to do so. It's uncertain whether a 6th gen fighter will carry a gun. See: F-X and F/A-XX programs, replacements for the F-22 and Super Hornet.

Citation for @MSalters claim: https://en.wikipedia.org/wiki/Air_engagements_of_the_Gulf_War#USAF_F-15E_vs._IRAF_Hughes_500

@Peter, No the first F-4 to have a gun in the aircraft (not an external gun pod) was the F-4E. And no Navy Phantoms ever had an internal gun. The German F-4F was a variant of the F-4E and had a gun, but I do not know of any other variant that had an internal gun.

I think that part of the reason that F-4s had guns was to attack lightly armored vehicles. The F-4 had just one 20mm gun which isn't really enough to shoot down another jet aircraft. The 20mm was used on ground targets, which could be destroyed or at least damaged with the 20mm.

@JLopez, No, the. gun was added for shooting down hostile aircraft. There were 42 M61A1 20mm gun kills during the Vietnam War,  15 of which were from F-4s.

I guess it's for the same reason that soldiers carry hand-guns.
They designed the F-4 without a cannon and added them back in 1965.
If you do go up to intercept a plane, if you do ever get close to it, what then? The minimum range of a sidewinder is 2.5 km.
This paragraph says,

Starting with block 50 (as far as the F-16 is concerned anyway), provisions have been made to fire the new 'hotter, faster, farther' PGU-28 round. It reputedly travels three times as far as the standard M53 round, effectively closing the gap between the Sidewinder minimum engagement range and the gun's maximum engagement range.

Up-close and personal is the norm in peace-time: e.g. the Chinese buzzing American planes n the China Sea; or NATO planes flying to meet Russians in the North Sea or off Alaska. What would you do without a short-range weapon? You would have to:

Keep your distance (e.g. run away if the enemy comes too close)
Shoot before they get too close (a huge over-reaction)
Be unarmed/disarmed at close range (not exactly "armed forces" then, is it).

Theoretically, the new round doesn't so much "close the gap" as provide an overlap. The nominal effective range of the Vulcan with M53s is about a thousand feet, which is about the AIM-9's minimum range.

I may be naive, but to me it also seems like a good investment. Most guns won't cost as much as rockets. If they end up being used at some point and save one 100 million dollar jet in one instance, great.
You end up not having too many rockets on those things, do you

You can't spoof a bullet with countermeasures.
The plain fact is that bullets cannot be diverted or fooled, unlike missiles, for which there are flares, chaff and different electronic countermeasures and early warning systems. It' s also much easier to destroy a plane with bullets now than it was a few decades back (during WW1 and 2 for instance). Unless your aircraft has some serious armor plating to guard against explosive  and incendiary rounds which are quite common, it's difficult to guard against bullets

It would seem likely that enemies which develop countermeasures might keep them secret until they can benefit from the surprise of using them.  The presence of guns would seem to offer some insurance against the possibility of encountering an enemy against which we have no effective weapons.

I see a lot of comments to the effect of "can't jam a bullet" while true, let's not forget that cannon targeting systems CAN be jammed. This reduces the pilot to having to rely on manual aiming, which would be ridiculously difficult for someone who has always relied on their radar-augmented targeting system showing them where to shoot, but still doable.

Nice point and also a reason why we don’t believe in computers to do the right thing, instead we trust pilots. When it becomes serious, it necessary to do things manually.

Better to ask a combat pilot I would think. However, a few points:
The internal gun for the F-35A, including a full load of ammunition, is less than 500 pounds - this represents about 2.5% of the F-35's total weapons payload capacity. That doesn't really sound too bad. They can save 80 lbs if they choose not to load it.
The external gun, which is what the VTOL/Naval B and C models get, is a few hundred pounds heavier than that, but optional.
Six seconds is actually a lot of 'gun time' for modern air-to-air systems considering speeds, targeting systems and lethality of the projectiles involved. It's not like WWII anymore where you'd slowly pull up behind a bomber and then have to spend several seconds pumping a bunch of crappy .50-cal lead in, hoping to poke enough holes in an oil cooler to make a difference.

A long time ago I used to build fighter aircraft (yes, with my hands). Anyway, the designers whispered behind their hands that the guns on this particular aircraft were there to make the pilots feel happy. The gunfire vibration tended to destroy the delicate instruments, so they were strongly discouraged from ever firing them.
In those days the designers (I was aspiring to be one) said, only half joking, an aircraft is a platform for delivering missiles or bombs, so it is basically a radar and a missile management system. The rest is infrastructure.
So in folk-parlance pilots like to have guns, in the same way as the air force likes to have pilots - it makes them feel better but is not strictly necessary.
edit: chatting to my old colleague about this, he said he offered to give a pilot a loudspeaker in his helmet going "Da-Da-Da-Da-Da", since it would be just as much use and a lot less weight. But the pilot was unimpressed.

Electronics certainly were delicate back then, so the designers had a point. Today, however, the gun vibration should be manageable. Specially designed circuits can survive being shot from a tank's gun with more than 10,000g peak acceleration.

@Graham Actually that is the best solution. Changing the tail design that far in the R&D cycle is going back to the drawing board. I suspect someone in management spent time designing military land vehicles. Same chassis; Different guns, and find a way to fit that cannon into it.

@The problem wasn't the tail per se, it was that they'd been told to fit the countermeasures to the aircraft body. But they went ahead and bolted them on the tail instead, with results which they'd been warned about.

As missile guidance and tracking systems evolve, so do countermeasures and maneuvers to cause a missile to miss. Planes don't carry that many missiles to begin with, and as mentioned before, missiles also have a minimal range. For all of these reasons, guns are still the only way to hit a plane with modern countermeasures. After all your missiles went for bust, or you're closer than the minimal operating range of your missiles. Turning away to gain distance is usually not an option, because as you increase your distance from the enemy, you open up an opportunity for them to turn in and engage you, not to mention losing airspeed is still generally considered a blunder in air-to-air combat.

While missile and radar systems designs get better they still cannot make a complete positive identification between friend or foe.  So you need to get close enough to visually ID the target.  Now you are in a dogfight situation with distance getting too close for missiles and too far for a gun.  So you need both.  You also still need to train pilots in close in dogfighting techniques and tactics.
All of this is complicated by the fact that aircraft are sold to many countries.  So the same aircraft may be flown by both sides of a conflict.  So now you need to get very close to see markings and other distinguishing characteristics.

The short and sweet answer:  Guided missiles have yet to demonstrate a Probability of Kill (Pk) or effective operating envelopes for all combat situations which can be entered into and a gun can fill those gaps well.  Gunfighting has certainly been viewed with disdain by Pentagon planners who have been repeatedly trying to do away with them in fighters despite the record of missiles in combat.   While improvements are constantly made to guided missile designs, the gun appears to be an integral component of era combat well into the future.
Also NEVER CONCLUDE THAT DOGFIGHTING IS OVER.  ACM is and always will be the fundamental and ultimate form of air combat.  The beginning and the end of the process.  I don’t give a hoot in hell what some idiot F-35 fanboy or defense OEM brochure or promo video has to say about that.  History is on the side of the dogfight.  Ignore that at your own peril.

I agree with you on how missiles are not effective at all ranges, but they are still very effective at most ranges. However guns are not very effective because dogfights are a thing of the past. As much as I love the traditional dogfight, modern air combat happens with both planes flying very far away from each other, and even if they are close the gun is of no use because it does not carry enough rounds and it does not do enough damage. Even a skilled pilot would miss many shots and the ones they hit wouldn't be enough.

If you think the dogfight is over, I’ve got some swamp land in Arkansas to sell you for $20 million an acre.  Missiles fail at critical times.  ROEs prohibit taking a shot without a visual ID on the bogey, stealth aircraft vs stealth aircraft prohibit such attacks, etc.   There are many reasons why reliance on pure missile air to air warfare is fallacious.

Traditionally, fighter aircraft are armed with guns. The gun is a versatile weapon, effective at short ranges against both air and ground targets. The idea of using a $150,000,000 aircraft to strafe the roads seems highly questionable, though. In the World Wars, a fighter was very much an expendable asset. Today that's no longer the case.

Assume you don't mean the actual road- too easy to fix, but rather road vehicles.

When radar stealth technology came to prominence in the 1980s there was widespread speculation that air combat would revert to a form similar to that seen in the First World War. This prediction has not been fulfilled, partly because the adoption of radar stealth technology has been quite slow, and partly because of continuing improvement in the effectiveness of air to air missiles.

The development of optical wavelength missile guidance systems, such as that used in the AIM9R Sidewinder is also a factor to be considered here, since these work well against targets with low IR and radar signatures.

I would also suppose that cannons/guns would be useful against ground targets in certain situations, e.g. tanks or artillery installments.
For example, the A10 Warthog's bread and butter is its main cannon. Though this plane is primarily designed with the elimination of ground targets in mind.
(As someone noted the A10 is an attack aircraft. I'm merely noting this aircraft's reliance on its cannon for ground attacks as a justification for the above supposition. Fighter jets may occasionally be in situations where attacking ground targets is necessary.)

Welcome to Stack Exchange. Please note that we prefer answers that either include reasoning or are backed up by solid sources. "Some anonymous person on the internet thinks it might be for shooting at targets on the ground" doesn't really help very much.

It is not the primary role of a fighter to attack ground targets, especially not with guns. Note that the  A10 is not a Fighter jet, but a ground Attack aircraft.

But most of the reason for having ground-attack aircraft is that fighters and fighter-bombers are not well suited to that role.

In theory, the F  35 is supposed to replace the warthog in a ground assault role eventually. Most modern fighters are multirole, and the ability to dogfight (which is where canons are useful) and the ability to strafe may be useful.

@JourneymanGeek But isn't that, like the F-15E, primarily air-to-ground with bombs, whereas the A-10 is air-to-ground with the cannon.

authors: Bob Jarvis - Слава Україні


text:
Why does the US President fly in an obsolete 747-200?
The Boeing VC-25A ("Air Force One") is a heavily modified 747-200. The two current airframes (SAM 28000 and 29000) were put into duty in 1990 and are slated to be replaced in 2017, although I suspect that latter date is optimistic.
These two 747-200s are among the last remaining 747-200s in service. I suspect that they are the best-maintained 747-200s in the world and they have considerable sunk costs in their custom rigging, but even when the aircraft were put in service in 1990 their model had already been replaced by the 747-300 (1982) and 747-400 (1989).
Why hasn't the VC-25A been replaced to keep up with the times? A 747-400 airframe is lighter, faster, and bigger (let alone a 747-8). These would all seem to be suitable features for what is most probably the most important aircraft in the USAF fleet. Is there some attribute of the 747-200 that makes it particularly well-suited for this role despite its obsolescence in the market?

The sunk cost you mention maybe the most important factor. It costs a fortune to modify two big 747 to the level of air force one. Also flight deck of -200 series is less susceptible by EMP compare to modern glass cockpit of later model.

@vasin1987 The VC-25 actually does have a glass cockpit. It's really, really modified.

For the sunk cost: It appears that the cost to modify the VC-25 was several times what a 747 cost at the time. The majority of the price was USAF-specific things.

IIRC, all 747s (excluding the 747-8 and the 747-SP have the same sized 231'8" airframes. Only the top deck is stretched from the 747-100 to the 747-400.

Bigger isn't really all that necessary for Air Force One. In fact, it may even be a hindrance in allowing it to land at smaller fields. As far as I know, the main reason the 747 was chosen for Air Force One was more its range than its size. The 777 didn't exist yet at that time. I wouldn't be at all surprised if the Air Force considers the 777 or even 787 to replace the VC-25s. I wouldn't be surprised if there are several airports around the U.S. where the only time they ever see something the size of a 747 is when the President is in town.

Ah, yes, the fabulous short-field performance of the 747!

Air Force One IS being updated and a new aircraft will be selected... http://www.nbcnews.com/watch/nightly-news/new-air-force-one-in-the-works-279218243658

Sometimes when i look at air force 1 it is a 747, sometimes a Gulfstream, sometimes a different airplane. The point is that AF1 is a callsign, not an airplane. Any plane carrying the president uses the callaign, not just when he's riding in a VC-25.

remember that AF1 is whatever POTUS is flying that's on the USAF registry. If he's in the back seat of an F-16D, that aircraft now uses call sign AF1. And the USAF has (or had, they may be retired) a few 747-400s, like the YAL-1.

I've done work on the VC-25, and as old as it is, the last thing anyone's going to utter on boarding for the first time is "this plane's obsolete."

For the record, the Pentagon has announced that it is replacing the 747-200s with 747-8s: http://us.cnn.com/2015/01/28/politics/boeing-air-force-one-fleet/index.html

@DaveDev Well, to be exact, it announced that the 747-8 is the only one that meets its specifications. They haven't actually ordered it yet. Honestly, I find the requirement that it be a 4-engine aircraft weird. I don't see why a 777-200LR/300ER or 787 wouldn't serve the purpose just as well at a cheaper operational cost and with access to more airports.

@reirab 4 engines == extra redundancy? I bet the president's laptop has 4 hard drives too!

@DaveDev The reason cited is indeed redundancy-related: If something goes wrong A 787 isn't taking off on one engine, but a 747 will be able to go on 3 (and can probably go on 2 in a dire emergency with adequate planning). If you need to move the president Right Freakin' Now you want the most flexibility you can get.

The 747-200 derivatives used for transporting the President of the United States are VC-25's operated and maintained by the United States Air Force. The US Air Force tends to keep planes for a long time, as they still operate 707 and B-52 aircraft twice as old as these 747's. The E-4 aircraft, also based on the 747-200, have been in service for 40 years. The reasons for keeping the VC-25 aircraft flying are similar to these other examples.
A lot of work went into the VC-25 aircraft. They have been modified to add aerial refueling capability, defensive mechanisms, and include extensive additional electronic equipment for communications. Installing the special interiors was also a major task. As with a bomber or AWACS, there is a large initial investment in development to design the systems and build the aircraft. Upgrading components and providing proper maintenance allows these aircraft to perform their missions long after their entry into service.
Another difference is the engines. The 747-200B model has GE CF6-50E2 engines producing 52,500 lbf thrust each. The VC-25 aircraft have newer CF6-80C2B1 engines producing 56,700 lbf thrust each, which is the same engine as the 747-300. Newer engines go a long way towards improved performance.
As DeltaLima notes, these aircraft do not spend nearly as much time in the air as commercial aircraft. Yet there are many commercial aircraft in service with similar or greater age, including some 747-200 aircraft.

The E-4 might actually be the reason for starting the VC-25 as a -200, when the -300 was out before VC-25 development started: the E-4 seems to have been introduced before the -300, so was based on the -200; when it came time to make the VC-25, the Air Force might have decided "we already have a -200 design with the necessary nuclear command post modifications (that AF1 also needs), so let's start from there."

@cpast: I think you're the only person to really answer the question, that is why was a -200 used in 1990 when it had already been superceded?

@cpast - I agree. I hope that fooot incorporates your comment into his/her answer, or that you spin it off as a separate answer if s/he doesn't.

My dad worked on Air Force One (Reagan, Bush 1, and part of Clinton) and we lived on Andrews when both 747's were received. I had the benefit of growing up being able to see the custom hangar built to hold both planes. As my dad was one of the engine mechanics, I can tell you that this is the most carefully maintained aircraft on the planet. Engines aren't fixed, they are stripped and rebuilt. Much more frequently than even the strictest requirements. I also have been on it a few times. Pretty magnificent from the inside. At least it was in the 90's.

Commercial aircraft are designed for a service life of about 25 to 30 years. The 747-200 is no exception. The VC-25A entered into service in 1990, so it has operated now for almost 25 year. The number of flights will be low compared to aircraft in commercial service, so the aircraft is not worn out at all. Replacing it with a new aircraft as soon as a new model is available on the market is wasting taxpayers' money.
In commercial service the Boeing 747-200 is too costly to operate these days. With a flight deck crew of three and four fuel-thirsty engines it is hard to compete with aircraft like the Boeing 777. But given the few flight hours that the VC-25A's make and the enormous investment needed to build a replacement aircraft the higher operating costs are acceptable.

On 28 January 2015, the US Air Force announced that the VC-25A's will be replaced by an aircraft based on the Boeing 747-8.

“The current fleet of VC-25 presidential aircraft has performed
  exceptionally well, a testament to the Airmen who support, maintain
  and fly the aircraft,” Secretary of the Air Force Deborah Lee James
  said. “Yet, it is time to upgrade. Parts obsolescence, diminishing
  manufacturing sources and increased down times for maintenance are
  existing challenges that will increase until a new aircraft is
  fielded."

Sort-of reminds me of the fact that it makes sense to keep old tungsten bulbs in your attic if you go there once a year. The fact that the new low-energy fancy bulbs were introduced can't change it. The same goes for the car you have in your garage and you take it out only on Nov 2nd to go to the cemetery. :)

Also, airlines operate their planes with the intent of making a profit. VC-25s are not operated to make a profit; there is much less pressure to reduce cost.

airliners are designed for a service life of a specific number of rotations and flight hours, not years. Of course those tend to lead to roughly the same number of years for most types, as most operators have similar usage patterns for their aircraft.

Since when has the military been concerned with wasting taxpayer money?

A lot of work went into configuring a 747-200 model into an airplane which can be called "Air Force One" (remember it is just a call sign):

a cabin with a kitchen and sound-proof conference rooms
Secure communication to satellites and military controls
Flares and air-refueling probes
etc.

In short, it is just a massive amount of work.
It is not like you have to replace everything once it is obsolete. For example, a typical consumer computer or smartphone can last quite a few years.

A computer / smartphone:

faster, no new essential functionality, brand new

An existing computer / smartphone:

slightly slower, 100% operational, 9 months old

No way I will spend significant time and money just for a slightly faster performance.
The VC-25 aircrafts are heavily customized for their task. Their components are well maintained. In aircraft lift cycles, 20 years is not short, but not long either. Surely the aircrafts will age over time, at one point it will be too expensive to maintain them than to order a new one. But such time has yet to come.

"No way I will spend significant time and money just for a slightly faster performance." Many iPhone buyers appear to disagree with you.

@MichaelKjörling Even there, I would say it's just a rather small group that actually upgrades on each cycle. Most wait at least a few cycles before upgrading (unless, of course, they manage to break their phone first.) There is a group that will buy the next device just because it's the next device (often without even fully understanding how similar the hardware is,) but it's a relatively small group. It's also worth pointing out that an aircraft qualified to be Air Force One is roughly 1,000,000x the price of an iPhone.

While this question already has an answer as to why the VC-25 has not been replaced yet, it does not address the question of why the VC-25 was based off the 747-200 in the first place.
As you mention, the aircraft entered service in 1990, however its first flight was back in 1987. This was before the -400 took flight, so this could not have been considered. The VC-25 featured a lot of modifications, none of which had been done before on a 747. A long period of planning would need to go into this, so the program launch and initial order would probably have begun a good few years before this date.
You are therefore looking at an order date of around 1985, shortly after the introduction of the -300. The -300 offered a number of benefits over the -200, such as greater capacity and improved operating economics. Capacity would not be of much benefit to the VC-25 program.
Given the timescales, the -200 would be nearing the end of production, so these would have been some of the last -200s off the line. Boeing may well have offered a good deal on these now superseded aircraft. The discounts offered would have gone a long way to offset any increase in operating costs, especially due to how infrequently the aircraft operates.
That is probably why the -200 was chosen. There was simply no benefit of operating the -300, and overall program costs would have likely been higher.

A new pair of 747-8s are planned to come in the next five years:

"The Boeing 747-8 is the only aircraft manufactured in the United States [that] when fully missionized meets the necessary capabilities established to execute the presidential support mission," said Air Force Secretary Deborah James in a news release. The contract must still be negotiated. The cost was not disclosed but the Wall Street Journal estimated that the pricetag could be $1.65 billion for two replacement 747s.
The current four-engine 747 and its backup are 25 years old, and Air Force officials consider the aircraft's normal life span to be 30 years. The new planes – a primary and a backup – are supposed to be ready in five years, James said.

Here is information straight from the U.S. Airforce:

Analyses of the capability requirements conclude a four-engine, wide body aircraft is required to meet the needs of the Air Force One mission. Market research determined there are two four-engine platforms that could meet the requirements; the 747-8 manufactured by Boeing in the state of Washington, and the A380 manufactured by Airbus in Toulouse, France.
The decision, made official through a Determinations and Findings document, authorizes the commercial aircraft purchase by other than full and open competition. This decision, in conjunction with the notification of the Air Force’s intent to award a sole source contract to Boeing for the modification of the 747-8, allows discussions with Boeing that will likely lead to a contract for the aircraft platform as well as the modifications necessary to missionize the aircraft.

However, supposedly B777 was also considered.

This would seem to indicate that there's something about the 747 design (4 engines?) that makes it preferable. Either that, or they wish to go with a known-known rather than the unknown-unknowns of another design.

Here is some more information. The article specifically mentions 4 engines as a criteria, and adds that the A380 was not likely to be chosen as it is built in France. I wonder how much consideration was given to the Il-96 and Y-20.

@dotancohen I would guess approximately zero. :) Also, if they wanted to use a 4-engine military transport, we have lots of those that are built in the U.S. (C-5, C-17, C-130, etc.) Now I can't help thinking about the President flying around in a B-52. - lol - Best of all options, though, would be to un-mothball a couple of SR-71s. Who needs missile defense when you can simply outrun any missiles fired at you?

I think that the living quarters in the Blackbird would have been a bit cramped. Also, the refuel-after-takeoff-and-warmup would complicate landing at most of the worlds airfields for sure!

@dotancohen haha, yeah using the Blackbird would have some... operational challenges.

The SR-71 would be the ideal choice, however, when the president needs to be over yonder, right freakin' NOW.

The 747-200 is commercially obsolete: in order to make a profit, an airline operating one would have to charge too much money to be competitive against someone with a 777 or a 747-800.
The Air Force isn't making money off of its VC-25 fleet.

many countries use their government aircraft as flying advertising billboards for their aviation industry. Of course in most of those countries that industry is effectively (or actually) nationalised. US might well be the exception there, leading to commercially obsolete types to be used in flagship functions by government agencies for longer than in say France or Russia.

@jwenting Does this actually apply to any countries other than Russia and France? Do any other countries actually have a long-haul airliner aside from Russia, France, and the U.S.?

@reirab Dutch government has always used Fokkers, even now that Fokker no longer exists... It's not just long haulers, it's at every level. Most diplomatic services will insist on using cars from their home country for embassies for example, even if there's no dealerships in the host country.

One reason the 747 was chosen rather than wait for the 777, is that it had 4 engines (like the previous 707).  Newer planes tend to have 2 engines as it's more fuel efficent.  4 engines can be more reliable when in a hostile environment.

Hello, and welcome to the Aviation Stack Exchange. You may note that the question refers to the 747-200 vs. other versions of the 747. Also, if you can provide references for your answer, that's always helpful!

Waiting for the 777 would have required, at an absolute minimum, a delay of seven years (the VC-25s first flew in May 1987; the 777 in June 1994). Regardless of the number of engines, it seems unlikely that the Air Force would have gone with what was then a completely new, unproven design. Imagine, for example, if the President's plane had a 787-style battery fire.

As a former RAF pilot has pointed out, when the Army decides on a new rifle the amount of disruption and retraining per involved person is small.  When the RAF decides on a new aeroplane, the amount of disruption and retraining per involved person is enormous.
Thus it pays not to replace aircraft without actual need.

Change doesn't necessarily bring improvements that are important to the mission. Do lightness, speed and size increase the odds of getting the President safely to his destination? They might, but the difference might not warrant the changes, especially when considering the downsides of change.
AF1 is a known entity with known risks, mitigated over many years. After all this time, everything about the aircraft is so well known that flights have a very low risk profile. Everyone from flight crew to ground crew to security personnel have done this a thousand times, raising the odds of success in any given emergency. Introducing any change likely adds risks that outweigh a bit of fuel economy.


text:
Why didn't Boeing make the Sky Interior for the Boeing 777?
Generally, Boeing has attempted to make the interiors of their aircraft as similar as possible, with one "unified" interior across different aircraft (i.e. based on a single "look and feel" for you UX enthusiasts), such as the older (but still fairly new) "Signature Interior" (debuted with the Boeing 777) and the newer "Sky Interior" (debuted on the Boeing 787).
When a new interior style was created, it seems like Boeing generally tries to copy that same interior to other aircraft. For instance, the Signature Interior was also used on earlier 737 NG and later 747-400 and 767 aircraft (with a retrofit offered), and the Sky Interior was used on later 737 NG (conflicting information about retrofits), 737 MAX, and 747-8.
While it seems from the above that Boeing is pushing new interiors onto older aircraft, including the 737, 747, and 767, this doesn't seem to be the case for the 777; even those coming off the assembly line today still have the original Signature Interior. Why didn't Boeing make a Sky Interior for their 777?
To make it clear what I'm talking about, here are some images and descriptions of each interior:
Signature Interior

Source: Airliners.net
Notes:

Middle section overhead bins and passenger service units appear integrated with the ceiling.
Side passenger service units are angled.
Overhead bin open handles can only be pulled one way.
The interior fittings are beige.

Sky Interior

Source: Lufthansa Magazin
Notes:

Middle section overhead bins and passenger service units appear attached to, rather than integrated with, the ceiling.
Side passenger service units are almost completely horizontal.
Overhead bins have open handles that can be pulled either way.
The interior fittings are white.

Keep these mind's answer points to a Ukraine International Airlines marketing material that advertises the "Sky Interior" for their 777s. However, clicking through the link shows a picture of their 777, but it has the same Signature Interior as above.

Note that the 777X series will have either the Sky Interior, or something very close.

I observe that the "Signature" picture appears to show First or Business class, whereas the "Sky" picture clearly shows Cattle class.

@MartinBonner The interior fittings I refer to in the question don't include the seats.

I don't know if it is an exception, but Ukraine International Airlines seems to claim that they have a Sky Interior in their 777. User gparyani comments that this claim may be mistaken. I'll leave this answer here in case somebody else independently comes up with the same thing.

Welcome on board modern Boeing 777-200ER airliner! The aircraft’s quiet and cosy cabins are equipped with comfortable reclining seats, contemporary Sky Interior [...]

https://www.flyuia.com/ua/en/services/long-haul-b777

Scrolling down towards their images shows the standard Signature Interior, not the Sky Interior.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/